Electronic spectroscopy of thiones

The phosphorescence excitation spectra of two thiones, 4-H-1-xanthione (XT) and 4-H-1-pyrane-4-thione (PT), cooled in a supersonic jet were investigated. The vibronic lineshape of the T1z origin of PT measured by cavity ring-down spectroscopy is considered and the excited state rotational constants are calculated. For XT the 3A2(nπ* ) → X1A1 phosphorescence excitation spectrum was investigated in the region 14900-17600 cm-1. The structure observed is shown to be due to the T1← S0 absorption and an assignment in terms of the vibronic structure of the band is proposed. A previous assignment of the S1 ← S0 origin is considered and the transition involved is shown to be most probably due to the absorption of a vibronic tiplet state T1z,v7. An alternative but tentative assignment of the S1,0 ←S0,0 transition is suggested. In the case of PT the phosphorescence excitation spectrum was investigated in the region of the 1A2(ππ*) ← X1A1 absorption band between 27300 and 28800 cm-1. The spectrum exhibits complex features which are typical for the strong vibronic coupling case of two adjacent electronic states. The observed intermediate level structure was attributed to the coupling with a lower lying dark electronic state 1B1(nπ*2), whose origin was estimated to be ~ 825 - 1025 cm-1 below the origin of 1A2(ππ*)0. Consequences of the vibronic coupling on the decay dynamics of 1A2(ππ*) as well as tentative assignments of vibronic transitions 1A2(ππ*)v ← X1A1 are also discussed. In the T1z ← S0 cavity ring-down absorption spectrum of PT, the vibronic lineshape of the T1z origin is analysed. As the T1z line is separated from the T1x,1y lines by a large zero-field splitting it is possible to use an Asyrot-like program to calculate the vibrational-rotational parameters determining the lineshape. It is shown that PT is non-planar in the first excited triplet state and the lineshape is composed of a mixture of A-type and C-type bandshapes. The non-planarity of PT is discussed.

Lead oxide-modified TiO2 photocatalyst: tuning light absorption and charge carrier separation by lead oxidation state

Modification of TiO2 with metal oxide nanoclusters such as FeOx, NiOx has been shown to be a promising approach to the design of new photocatalysts with visible light absorption and improved electron–hole separation. To study further the factors that determine the photocatalytic properties of structures of this type, we present in this paper a first principles density functional theory (DFT) investigation of TiO2 rutile(110) and anatase(001) modified with PbO and PbO2 nanoclusters, with Pb2+ and Pb4+ oxidation states. This allows us to unravel the effect of the Pb oxidation state on the photocatalytic properties of PbOx-modified TiO2. The nanoclusters adsorb strongly at all TiO2 surfaces, creating new Pb–O and Ti–O interfacial bonds. Modification with PbO and PbO2 nanoclusters introduces new states in the original band gap of rutile and anatase. However the oxidation state of Pb has a dramatic impact on the nature of the modifications of the band edges of TiO2 and on the electron–hole separation mechanism. PbO nanocluster modification leads to an upwards shift of the valence band which reduces the band gap and upon photoexcitation results in hole localisation on the PbO nanocluster and electron localisation on the surface. By contrast, for PbO2 nanocluster modification the hole will be localised on the TiO2 surface and the electron on the nanocluster, thus giving rise to two different band gap reduction and electron–hole separation mechanisms. We find no crystal structure sensitivity, with both rutile and anatase surfaces showing similar properties upon modification with PbOx. In summary the photocatalytic properties of heterostructures of TiO2 with oxide nanoclusters can be tuned by oxidation state of the modifying metal oxide, with the possibility of a reduced band gap causing visible light activation and a reduction in charge carrier recombination.